Field of the Invention
This invention relates to simplified methods for fabricating a catalyst coated membrane (CCM) for solid polymer electrolyte membrane fuel cells. The invention further relates to CCM structures comprising reinforcement layers of expanded polymer sheets and which exhibit improved mechanical strength. The improved CCM structures can be fabricated using the simplified methods of the invention.
Description of the Related Art
A catalyst coated membrane (CCM) is a crucial component for solid polymer electrolyte fuel cells. A CCM is composed of an anode, a cathode, and a proton-conducting membrane ionomer layer (e.g. perfluorinated sulfonic acid) which serves as the electrolyte. The anode and cathode comprise appropriate catalysts and are bonded in layer form to the membrane ionomer layer. During operation of the fuel cell, the anode facilitates the conversion of fuel (such as hydrogen) to electrons and protons. The generated protons pass through the membrane ionomer layer, while electrons are forced to flow through an external circuit. Finally, protons, electrons, and oxygen react at the cathode to form water.
Among the many known methods for preparing CCMs, decal transfer methods are probably the most commonly used. In this approach, anode and cathode catalyst layers are pre-coated separately onto supporting substrates, which is then followed by a hot bonding process that laminates the two catalyst layers to the electrolyte membrane. The laminating of the catalyst layers can be done either simultaneously or sequentially. Although decal transfer methods have merits, they also have several disadvantages. First, a decal transfer step requires the use of a laminator, which adds extra cost to the processing. Second, the integrity of the membrane ionomer layer can be compromised during the hot bonding process, especially when the ionomer layer is thin (e.g. <10 μm) and the catalyst layers are rough. Third, defects such as wrinkles can be introduced in the assembly during decal transfer processes, which can significantly reduce manufacturing yield and thus increase cost.
Much effort has been devoted to address the aforementioned issues with decal transfer. For instance, direct coating of the catalyst layers onto the electrolyte membrane via various coating techniques (such as spray coating or inkjet printing) has been adopted to avoid decal transfer steps. However, the solvents (e.g. H2O and alcohol) in the typical catalyst inks significantly swell the electrolyte membrane during coating and this leads to significant cracking of the catalyst layers during subsequent drying, thereby compromising the integrity of the membrane-catalyst interface.
More recently, a new approach has emerged, which allows the direct coating of catalyst and ionomer layers on top of each other. WO2013/064640 discloses an “integral” approach to first coat the cathode layer onto a supporting substrate, followed by electrolyte membrane coating, in which an expanded polytetrafluoroethylene (ePTFE) substrate pre-impregnated with ionomer dispersion is introduced and then adhered to the cathode layer. Finally, the anode layer is coated onto the membrane ionomer layer to fowl the CCM. In this approach, only one ePTFE sheet is used for mechanical reinforcement of the CCM. No data regarding the mechanical strength and hydration stability of the CCM is disclosed. In other approaches, more than one reinforcement layer may be employed in CCM fabrication. For instance, US20130202986 discloses a fuel cell construction comprising a reinforced electrode assembly comprising first and second porous reinforcement layers.
An important requirement for commercial fuel cell stacks is long-term durability. In automotive fuel cell applications, it is typically required that stacks should be able to operate a minimum 5,000 cycle hours (equivalent to 150,000 miles of driving) in order to compete with present automotive internal combustion engines. The mechanical strength and the in-plane hydration stability (i.e. the dimensional stability of the CCM in the planar directions as a function of hydration state) of a CCM have been identified as two major factors affecting its durability. One important approach to achieve high mechanical strength and low in-plane swelling (when hydrated) in a CCM is to introduce a reinforcement layer in the middle of electrolyte membrane (e.g. as disclosed in U.S. Pat. No. 5,547,551 or EP1998393). The swelling of the ionomer layer can be constrained by the reinforcement layer. Among the possible reinforcement materials, expanded PTFE (ePTFE) has been widely used. Desirably, ePTFE can be manufactured in a continuous web which is characterized by a machine direction (MD) and a transverse direction (TD). For use as a reinforcement material in a CCM, the properties of the ePTFE should preferably be tailored to provide high in-plane mechanical strength in both the MD and the TD in order to minimize in-plane swelling in both the machine and transverse directions. In addition, the presence of an ePTFE reinforcement layer should preferably have minimal impact on proton conductivity over the electrolyte membrane and fuel cell performance generally. Unfortunately, ePTFE with preferred properties is not readily available. Most commercial ePTFE web products are anisotropic, particularly in that the mechanical strength in the machine and transverse directions are quite different. This leads to anisotropic mechanical strength in a CCM and thus greater swelling in one direction (i.e. an uneven swelling ratio between the MD and TD).
There remains a need to develop improved CCMs with balanced mechanical strength in both the machine and the transverse directions. Further, there is a need to simplify the preparation process by preferably avoiding the use of decal transfer processes. The present invention fulfills this and other needs.